In the showroom, I found the 541i to be a substantially more lifelike reproduction, through the same high-quality amp and speakers.
I purchased a 541i, to replace my aging Marantz CD63-II and can't say I'm overwhelmingly satisfied. It is more rhythmic than the CD63 and has much better bass, but I was hoping for a more involving experience of the music, such as with my vinyl (Linn LP12+Valhalla+Cirkus,Profile-II arm,Benz MC-1 cartridge,Rotel RQ970BX phono stage), and that simply hasn't happened.
I'm using the 541i alone as well as into a Creek D-A converter, and find the differences less obvious than was the case with CD63. The 541i does sound better with the separate D-A than without.
What I do find is that the 541i sounds significantly better at low volumes than at realistic volume levels.
So my answer to your question is: yes there is a substantial difference between the 521i and 541i, but the 541i hasn't done it for me.